Sputter coating is a widely used technique for depositing a thin film of material on a substrate. In a sputtering deposition process ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target of coating material at the cathode by an electric field causing atoms of the target material to be ejected from the target surface. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material.
In operation, when the sputter gas, e.g. argon, is admitted into a coating chamber, a DC voltage applied between the cathode and the anode ionizes the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged cathode. The ions strike the target in front of the cathode with a substantial energy and cause target atoms or atomic clusters to be sputtered from the target. Some of the target particles strike and deposit on the wafer or substrate material to be coated, thereby forming a film.
To attain increased deposition rates and lower operating pressures, magnetically enhanced cathodes are used. In a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate of coating material. Thus, the magnetic field causes the electrons to travel in a closed loop, commonly referred to as a “race track”, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability, thereby leading to a much higher sputtering rate than that obtained without the use of magnetic confinement. Furthermore, the sputtering process can be accomplished at a much lower gas pressure.
Typically a magnetron sputtering system is operated at a pressure of 2*10^−2 Pa−1*10^−1 Pa. during sputtering. To establish this pressure, typically the chamber is pumped down to a pressure of <1*10^−4 and a controlled flow of a gas, typically Argon (and in case of reactive sputtering Argon and Oxygen) is fed into the chamber to maintain the desired pressure. In the case of a diode system, i.e. when no magnets are used, a pressure of >2 Pa. is required to be able to ignite and sustain a plasma. High pressure has the disadvantage that the mean-free path is greatly reduced, which causes extensive gas scatter. This results in hazy coatings.
It is desired to create a magnetron sputtering system that increases coating rate and product uniformity across an individual substrate, from substrate to substrate and from run to run.
Cathode geometry, particularly the relationship between the cathode shape, position and dimensions and the objects to be coated, has a significant effect on the rate of deposition and the area coated, as well as product quality and consistency. Variation in layer thickness across a substrate is referred to as runoff. The runoff can be predicted through modeling. It is desired to provide good film thickness uniformity, low runoff, over large diameter substrates.
In many coating apparatuses masking is used to reduce the coating rate variation to acceptable levels. But over time the masks typically accumulate large amounts of coating material. Once the material on the mask reaches a critical thickness it may flake off and contribute to particles that compromise the coating quality. Also trimming and maintaining such masks are elaborate processes. Furthermore, as masks become coated, they gradually change their shape, which continuously alters the coating distribution. In some instances a significant portion of the sputtered particles is shielded which reduces the material utilization. In the prior art, heavy masking, blocking up to 40% of the coating material, is needed to achieve an acceptable thickness distribution (runoff) of +/−1.5% across a 100 mm wafer. A stable system is required to provide run to run uniformity. It is desired to provide a device that does not use a mask.
The anode provides a charge differential to the negatively charged cathode. This can be provided as simply as an electric charge provided to the chamber walls. However, the sputtered material is also deposited on any surface exposed to the sputtered atoms. If the coating is an electrically insulating material, such as a metal oxide, the build up of the material on other parts of the sputtering apparatus can cause problems. In particular, the build up of an insulating coating on the anode interferes with the ability of the anode to remove electrons from the plasma, as required to maintain the plasma's charge balance. This destabilizes the plasma and interferes with deposition control. Coating build-up will cause the anode location to move to another surface in the system. This instability affects coating quality. Numerous prior art anodes have been proposed to overcome the problems of the anode becoming coated with the coating material. Many prior art anodes function at very high voltages that also increase the problems of arcing, which damages coating quality. A low voltage anode that can provide a stable anode location is important to ensuring consistent coating quality.
An increase coating capacity can be realized through an increase in deposition rates or an increase in load size, or both. To increase deposition rates, it is necessary to increase the power density at the target. However, higher power density leads to an increase in arcing and in some targets, such as silicon, to an increase in target cracking. A larger target allows a higher material removal rate without increasing the power density. Greater efficiency of oxidation of the deposited film can also increase the deposition rate for reactive sputtering. Maintaining runoff limits is a challenge to increasing load size. In a concentric system where the cathode and planetary drive share a common center point, enlarging the planetary drive system for larger or a greater number of substrates requires an increase in throw distance. This increases problems of gas scattering by increasing the probability of particle collision—also called “a reduction of the mean free path.” The result is an increase in surface roughness of the coating, perceived as an increase in scatter or haze. It is desired to increase throughput to greater than 3600 cm2/hr for a 3 micron thick application while maintaining a runoff of +/−0.5%. For some industries capacity to coat a 300 mm substrate is necessary. It is desired to increase capacity without sacrificing coating quality. It is also desired to maintain a low temperature process despite increased power input in order to be able to process temperature sensitive materials.
It is an object of this invention to provide a ring shaped cathode for use in a magnetron sputter coating device having a geometry that provides rapid coating over a large surface area while maintaining high coating quality and minimizing material waste.
It is a further object of this invention to provide a magnetron sputter coating device including a ring shaped cathode geometry that produces high quality coatings without the use of a mask.
It is an object of this invention to provide a magnetron sputter coating device incorporating a ring shaped cathode in combination with a low voltage anode vessel.
It is an object of this invention to provide a ring shaped cathode for use in a magnetron sputter coating device that incorporates an anode vessel at the center of the cathode ring.
It is a further object of this invention to provide a ring cathode for use in a magnetron sputter coating device that incorporates a reactive gas outlet at the center of the cathode ring.
It is an object of this invention to provide a ring shaped cathode for use in a magnetron sputter coating device that incorporates an anode vessel at the center of the cathode ring that can deliver an activated reactive gas.